Continued advances in the design of electronic devices for data processing and communications systems have placed rigorous demands on the design of electrical connectors. Specifically, electrical connectors having higher densities and pin counts are needed for design advances which increase integration of solid state devices and which increase the speed of data processing and communication. Designing connectors to have higher densities and higher pin counts requires careful consideration of the problems which result from decreasing the distance between contacts. This is particularly true in applications involving daughter board/back panel interconnections where the connector functions to establish an electrical connection and functions to mechanically hold the daughter board in position.
Density and pin count are often viewed interchangeably, but there are important differences. Density refers to the number of contacts provided per unit length. In contrast, the number of contact elements that can reasonably withstand the mating and unmating forces is referred to as the pin count. As more functions become integrated on semiconductor chips or on flexible circuit substrates and more chips are provided on printed circuit boards (PCBs), each PCB or flexible circuit must provide more inputs and outputs (I/Os). The demand for more I/Os directly translates to a demand for greater density without sacrificing electrical or mechanical performance, particularly when such devices and integration techniques are utilized in devices having back panels.
The importance of electrical performance of high density connectors was recognized in U.S. Pat. No. 4,824,383--Lemke, incorporated herein by reference. This patent proposed designs for plug and receptacle connectors for multiple conductor cables or multiple trace substrates. In such designs electrical performance was assured by electrically isolating individual contact elements or groups of contact elements through the use of conductive walls to prevent or minimize crosstalk and signal degradation. Although, the connectors disclosed in U.S. Pat. No. 4,824,383 increased contact element density, industry driven density demands continued to grow. U.S. Pat. Nos. 5,057,028--Lemke et al. and 5,169,324--Lenke et al. (now U.S. Pat. No. Re. 35.508), all incorporated herein by reference, disclose two row plug and receptacle connectors for attachment to printed circuit boards (PCBs), which connectors again exhibited good electrical performance through the use of conductive walls to provide isolation.
Electrical performance for high density connectors was also the focus of U.S. Pat. Nos. 4,846,727, 5,046,960, 5,066,236, 5,104,341, 5,496,183, 5,342,211 and 5,286,212. These patents disclose various forms of stripline structures incorporated into a plug and receptacle system. In a high density stripline structure, columns of contact elements arranged in a side-by-side array with conductive plates disposed between each column. The connectors are designed so that the plug and receptacle ground plates contact one another, thereby providing isolation between columns. A further aspect of this system is the modular design of the receptacle. Each column of receptacle contact elements is formed by molding the contact elements into a frame of dielectric material. One of the problems with these types of connectors is that the use of conductive walls for electrical isolation requires space. In certain applications the space or volume necessary for such isolation is impractical.
In those applications where space is critical, proposals for achieving desired electrical performance have included the use of high density connectors in which one pin, selected for signal transmission, is positioned between pins connected to ground. Such patterns are known as interstitial arrangements. Such contact element patterns are suggested in U.S. Pat. Nos. 5,174,770, 5,197,893 and 5,525,067. It will be appreciated that while such isolation schemes can be implemented in more compact connectors, such schemes require greater numbers of pins than were available in the previously described connectors using conductive walls.
In back panel applications, increasing the number of pins has a direct impact on mechanical integrity. As the number of pins increases, the number of bores or through holes in the back panel and daughter board increases. As the number of bores in a printed circuit board increases while at the same time decreasing the distance between each bore, as will be the case in high density applications, the mechanical integrity of the board decreases. As mechanical integrity decreases the ability for the back panel to mechanically hold the daughter board in position decreases.
It will be appreciated that in back panel applications, the daughterboard is held in position, i.e., vertical and horizontal orientation, sometimes exclusively, by the connector used to electrically interconnect the two. Daughterboard size, the number of daughterboards and the components mounted to the daughterboards combine to produce the stresses and moments acting on the back panel after assembly. If the amount of back panel material is reduced in particular locations due to larger numbers of closely spaced through bores, back panel failure can occur, i.e., the back panel could deform or even break.
One might conclude that the solution to back panel mechanical failure would be to use surface mount techniques to establish electrical connection to the back panel. Since surface mount techniques do not require the use of through holes, such techniques would afford the ability to connect high density connectors to the back panel without impacting mechanical integrity. Unfortunately, such solutions would be unsuccessful.
Surface mount techniques typically involve temporarily fixing a component to a printed circuit board using a paste. After pasting, the board and temporarily fixed components are heated in order to reflow solder material previously coated onto the leads of the surface mount components. In back panel applications, numerous connectors are attached to the back panel board, which is typically a relatively large circuit board. In order to assure adequate reflow, thereby establishing good electrical connection, for numerous components spread over a relatively large board, the back panel board would have to be subjected to significant heat. Unfortunately, heat which is too high or too long in duration can actually interfere with establishing good surface mount terminations. Consequently, surface mount techniques do not form the answer to the need for higher density connectors for back panel applications.
Consequently, a need still exists for a connector system which maximizes the number of contact elements available for ground/signal assignment and which does not jeopardize either the electrical or mechanical integrity of back panel applications.
One other consideration which must be taken into account when designing high density connectors, particularly for back panel applications is the design of the structure for attaching the receptacle portion of the connector. One important factor in receptacle attachment is alignment. When a single receptacle contact element is misaligned, insertion force increases a negligible amount. However, when misalignment occurs in a high density contact receptacle, insertion force could increase to unacceptable levels. In other words, if misalignment occurs during the mounting of a receptacle to a circuit board, it may become impractical for the board to be mounted to a plug or vice versa.
Consequently, a need still exists for a high density connector system which provides sufficient alignment after mounting such that insertion force remains within acceptable limits.